Acute pyelonephritis is one of the commonest infectious human diseases. One of the most prevalent pathogens of acute pyelonephritis is Escherichia coli. The infection is often associated with urinary tract obstruction, pregnancy, or malformation of the urinary tract. Presence of urinary stones, underlying pathological alterations of the kidney, and possibly metabolic disorders such as diabetes mellitus, are predisposing factors to acute pyelonephritis. Despite the commonness of acute pyelonephritis, little has been understood about the mechanisms of renal tissue destruction occurring in this disease. Escherichia coli is a normal intestinal flora of humans, and is weakly pathogenic. Because of the low virulence of E. coli, it occurred to us that the renal tissue damage seen in Escherichia-coli-induced acute pyelonephritis may be partly due to leukocytic proteolytic enzymes. To test this, a sequence of morphologic events of the kidney infected with E. coli will be studied in the following: (1) rats that have impaired generation of chemotactic factors, (2) leukocyte depleted rats, and (3) rats that will be administered with enzymes that are inhibitory for proteolytic enzymes. Impairment of generation of complement mediated chemotactic factor will be produced by administration of cobra venom factor. Number of polymorphonuclear leukocytes will be depleted by injection of nitrogen mustards. Proteolytic enzymes will be inhibited by leupeptins, enzyme inhibitors of microbial origin.